I'm a freak I guess, I'm fine with a 4 color hires Workbench on AGA
There's Indivision AGA, but for bus graphics cards I think it's a mistake to add stuff and stay in the same size case. That's mostly because most RTG cards are so big, though. Some VRAM fastmem + VGAout could fit in the internal slot, but you'd lose acceleration to drive the higher resolutions. Possibly an accelerator with on-board VRAM, or (ab)using some port on an existing acceleration, but... even now when we can replace a harddisk with MicroSD, the case size would limit what can be done.
A1200 can go 1280x512 (+overscan), and maybe there is a 21:9 display with a scaling chip that can deinterlace it correctly. There's always 800x600@73 Hz, which should work on any VGA CRT with a minimum of fuss?
Friends who have RTG have A4000, two monitors and an autoswitcher. A pretty good solution actually.
I would say native or Indivision AGA is the best way if you're determined to stay with the A1200 and see what it can do